I think in many cases, if too much logic and reality is allowed to creep into horror and fantasy movies then the excitement is lost and there would be no movie.
In Wolf Creek, had the girls further shot the baddie in his lair or smashed his head in with heavy objects and killed him... no more tension.
Tom Savini's character 'Sex Machine' explains the situation with the vampires in From Dusk 'Till Dawn: "These vamps... they're strong, but they have soft bodies...".
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